The use of fuel-efficient internal combustion engines in vehicles has an effect on the vehicle air conditioning, however, in that, in certain operating ranges, e.g., at low outside temperatures during the starting phase, the amount of heat given off to the coolant is no longer sufficient to comfortably heat the vehicle. Auxiliary heaters are therefore necessary to ensure comfort at low temperatures and to defrost the vehicle windows if necessary. A climate control system can also serve as an auxiliary heater, especially since more and more vehicles are being equipped with a climate control system as a standard feature. At low temperatures, the climate control system is used as a heat pump by reversing the direction of coolant flow, whereby a xe2x80x9cgas condensing apparatusxe2x80x9dxe2x80x94which is part of a CO2 climate control systemxe2x80x94draws heat from the surrounding air when operated in the heating mode. The heat pump consumes relatively little energy and responds spontaneously with high heat output. A main disadvantage of such a heat pump lies in the fact, however, that ice forms on the air side of the gas condensing apparatus when outside temperatures are low. As a result, the amount of cooling air passing through the radiatorxe2x80x94which is usually installed downstream in the air streamxe2x80x94of the internal combustion engine is inadequate, so that sufficient cooling of the internal combustion engine is no longer ensured when output increases over the short term, e.g., when entering a highway.
A device and a method for heating and cooling a passenger compartment of a motor vehicle is made known in EP 0 945 291 A1. In the heating mode, the coolant is compressed by a compressor and travels via a 3/2-way valve to a passenger-compartment heat exchanger, where it gives off part of the heat produced by compression to the colder air inside the passenger compartment of the vehicle. The coolant flows from the passenger-compartment heat exchanger to an expansion device, in which it is cooled down to the extent that it can absorb heat from the surrounding air in a gas condensing apparatus located downstream. Additional heat could be supplied to the coolant in a downstream exhaust-gas heat exchanger that is acted on by hot exhaust gases from the internal combustion engine.
From the exhaust-gas heat exchanger, the coolant returns to the compressor, and the coolant circuit is closed. If the coolant is expanded in the expansion device to a temperature that is below the ambient temperature, the air passing through the gas condensing apparatus can be cooled down to a temperature below the saturation temperature. In this case, water condenses out of the inducted surrounding air. If the temperature is below the sublimation line of the water, the water changes to the solid state and ice forms on the gas condensing apparatus. Since the gas condensing apparatus is usually installed upstream from a radiator of the internal combustion engine in direction of air flow, proper cooling of the internal combustion engine is endangered if ice forms on the gas condensing apparatus. To prevent excessive ice formation, a bypass line is opened via a 3/2-way valve when critical ambient conditions exist, so that the gas condensing apparatus is closed briefly. The coolant bypasses the gas condensing apparatus and flows directly to the exhaust-gas heat exchanger and, from there, to the intake of the compressor. Furthermore, a process is feasible in which the circuit is then designed as a hot-gas process, whereby the compression heat of the compressor is then used exclusively as a heat source.
According to the method according to the invention, in the heating mode, the switching valve in the bypass line associated with the gas condensing apparatus is opened and the flow of coolant through the gas condensing apparatus is stopped as soon as a layer of ice that exceeds a limit thickness has formed on the air side of the gas condensing apparatus. As long as the air flow on the air side of the gas condensing apparatus has a temperature above the freezing point, the ice layer thaws and is removed by the air stream. The ice layer also evaporates when the temperature of the air stream is below the freezing point, however, so that the gas condensing apparatus can be usedxe2x80x94after some timexe2x80x94as a heat source once more. As a result, the run times in the heating modexe2x80x94when the gas condensing apparatus is not used as a heat sourcexe2x80x94are reduced to a minimum. The amount of heat that is available is therefore so great that additional heat exchangers, e.g., exhaust-gas heat exchangers, can usually be eliminated.
Since the resistance to flow of the gas condensing apparatus increases due to the layer of ice on the gas condensing apparatus, the pressure drop on the air side of the gas condensing apparatus can be evaluated as a measure of the ice formation at critical ambient temperatures. If, at critical ambient temperatures, the pressure drop exceeds a specified limit value, the coolant flow is directed through a bypass line by means of an appropriate electronic evaluation unit controlling one switching valve at a time in the bypass line and/or in the inlet or outlet of the gas condensing apparatus.
According to another possibility, a capacitive or resistive sensor is located on the air side of the gas condensing apparatus, the capacity or resistance of which changes as a result of the ice layer. If the signal from the sensor exceeds a limit value when critical ambient parameters exist, the coolant flow is directed through the bypass line. Critical ambient parameters include, in particular, low ambient temperatures and high relative humidity.